The folks at TPGuy have finally made a Firefox extension for ARC Toolkit:
addons.mozilla.org/en-US/firef…
I installed it yesterday. Nothing has caught fire. Appears to work as I recall it on Chrome.
ARC Toolkit – Get this Extension for 🦊 Firefox (en-US)
Download ARC Toolkit for Firefox. Accessibility testing tool from TPGiaddons.mozilla.org